This invention relates generally to the field of synthesizing metal borohydrides and more particularly to a process for synthesizing sodium borohydrides and its alkali solution.
The metal borohydrides especially alkali metal borohydrides such as KBH4, LiBH4 and NaBH4 are unique compounds in their ability to carry large amounts of hydrogen. They can be stabilized by dissolving into alkali solution. They can release hydrogen gas by the use of catalysts such as Cu, Co, Ni etc if needed. When reacting with water, 1 molar of KBH4 or NaBH4 can produce 4 molars of pure hydrogen gas in which 2 molars of hydrogen come from borohydrides and the other 2 molars come from water. The theoretical capacity of hydrogen storage in NaBH4 is high up to 21 wt % because water also becomes carrier of hydrogen in this case. Therefore, the metal borohydride alkali solutions offer great potential as a fuel in fuel cell systems or any other applications when such effect is desired. In addition, as atoms H in metal borohydrides are −1 value, which has excellent reducing characteristics, therefore, in theory, metal borohydrides especially alkali metal borohydrides should find their applications in almost every field where reducing agent is needed.
The metal borohydrides have not obtained their expected widespread usage simply because of their high costs. As said in U.S. Pat. No. 3,734,842, “the selling price of sodium borohydride”, a kind of metal borohydrides can be used to further produce other metal borohydrides, “produces in accordance with conventional industrial processes is necessarily pegged to the cost of metallic soduim”. The reaction now employed for the production of sodium borohydride in industry as below:4 NaH+(CH3O)3B=NaBH4+3 NaOCH3
One molar sodium borohydride needs 4 molars metallic sodium conventional industrial process. This process by the use of metallic sodium to produce metal borohydrides was described clearly in details in the article titled “Na borohydride: Can cost be lowered?”, Canadian Chemical Processing, 47, No. 12, 57-59 and 62 (1963) and U.S. Pat. No. 3,473,899. U.S. Pat. Nos. 3,734,842, 4,931,154 and 5,804,329 described another method to produce metal borohydrides by the use of electrochemical cells. In these kinds of methods, a cathode, an anode and membrane were used and borohydrides were said to be produced from borate ions directly. However, once borate ions are reduced to borohydride ions, OH ions are produced also and then negative borate ions and OH-ions will be co-existing in electrolyte, whichever the electrolyte is aqueous solution or not, OH− is prior to borate ions to be produced into water. This will result in a very difficult to further produce borohydride ions and even can be produced; the efficiency of making borohydrides ions will be too low to be practically used in industries. Therefore, in the past 50 years many efforts have been put in trying to reduce the costs of metal borohydrides but by now the production of metal borohydrides in industry scale is still pegged by metallic sodium. Consequently, the costs are still very high.